Borings are the main procedure used for soil exploration on site. There are different types of borings. They can generally be grouped into 3 types, they are:

• Trial pits of up to 3m deep
• Borings of up to 30m deep
• Headings and Shafts

Trial pits

Trial pits are pits of about 1.2m by 1.2 m. They can be dug with the use of labour or excavating machines. Trial pits should be dug  at distances of 20m apart and clear of the positions of the building’s foundation. Trial pits are economical up to a depth of 3m. Trial pits are suitable for small-scale works. The main advantage of this method is that soil and rock samples can easily be exposed and examined.

Deep boring.

This is the process by which hand or mechanical/machine boring tools are used to drill holes into the soil for investigative purposes. Deep borings are suitable for medium to  large scale works such as high rise buildings.

Hand or mechanical auger borings are cheap methods of boring soils which can stand unsupported. Holes can be sunk up to 3m. For soils that cannot stand on their own such as loose soils, steel casing can be dug surrounding the bored area.

A steel casing called shell is used to collect soil samples for testing.

It is done at regular intervals as hole is being dug.

For deeper borings of up to 30m, machine operated drilling must be employed. They are of many types. There is the percussion boring in which the soil formation is broken up by repeated blows after which water is added to the hole as the work proceeds. The water helps to wash out the soil debris by pressure washing or by shell auger. The samples collected by this method are disturbed samples because of the repeated blows to the soil formation.

Wash boring method uses only a strong jet of water or drilling mud to bore deep holes in the soil. This method produces undisturbed samples. However, it is suitable if the soil does not contain boulders or large gravels.

Rotary drilling is a method used for boring of soil with rock formations. The drilling bit is fitted with industrial diamond to break the rock formations. Drilling mud, water or compressed air is jetted into the drilled hole to flush out the rock debris.

Headings and Shafts.

Headings and Shafts are employed to explore steeply dipping strata. It is best suited for very large scale works such as dams, power stations etc. The method is used to create exploration tunnels. The advantage they have over pits drills is that they can be easily drained of water and also easy removal of soil debris. Shafts are bored using large power driven augers. The sides of the shafts must be supported to protect the work personnel engaged in soil inspection.

Choice of soil exploration methods.

The choice of different exploration methods will depend on the following factors:

• Topography.
• Nature of ground.
• Cost.

The construction of buildings, dams, reservoirs, docks and other types of construction works, soil investigation will require deep and closely spaced borings.

Borings will reveal the types of soil layers beneath the proposed foundation, and whether or not it will be suitable to carry the proposed structure safely.

Borings should be dug in a regular pattern in such a way that it gives a clear picture of all significant variations of the soil within the site.

Investigative boreholes should be at least 1.5m deep. Three main factors will govern the depth of exploration:

Depth to which the foundation load will act on beneath the base of the foundation.

Depth to which weathering will affect the soil.

Depth at which impervious strata occur.

Some investigative borings  can be up to 6m deep.

The standard depth for building structures is up to 1.5 times the breadth of the foundation.

How soil is stressed under foundation loading

Load of the structure is transferred to the foundation and then to the surrounding soil beneath the foundation. The soil stress beneath the foundation is reduced with depth.

The diagram above shows the soil stresses below a pad foundation. Notice that at 1.5b below the foundation level, the bulb stress is 0.2 of the original stress just under the foundation. This value is higher for a strip foundation as shown below.

The reason for this is because strip foundation is continuous while pad foundation is isolated.

Before spending money on practical on-site soil test/ investigation, it is good to  find out if these tests/investigations have been carried out in the past by Government agencies. checks may reveal records of previous, close-by soil investigations. Geological data, aerial photographs, historical information and local knowledge of the site area will help a great deal.

Obtaining preliminary data is best gotten from the relevant government agency or establishment. This will ensure that you are using standardized and government recognized data for your preliminary soil investigation.

Preliminary data may not provide all the necessary information you need concerning the  soil situation of your site, but it saves you the cost of starting an investigation from the scratch and at the same time providing recognized data for you to work with.

What is Site Investigation?

Site Investigation means the various investigations that determine if the site will be suitable for its intended use based on structural, economical and environmental factors. Site Investigation is not limited to preliminary works alone. It is also undertaking to investigate defects, failure or safety of existing works.

Types of site investigation

Investigation of site for new works:

The investigation could involve determination of the overall suitability of the soil for the construction work.

The economical and environmental impact the proposed new works will have on existing infrastructure.

Proper preliminary site investigation will help determine the correct financial implications, material as well as equipment  necessary for a successful project.

Investigation of defects or failure of existing works.

This type of investigation may be  mandatory to establish cause of failure and provide remedy. For instance, in the case of a building collapse after several years, soil investigation may reveal reduction of the bearing capacity of the soil which may have been caused by changes in level of ground water.

Investigation of the safety of existing works.

When proposed new works are being planned, it will be necessary to assess the impact it will have on existing works or infrastructure. Structurally, situations that may be investigated are;

Will excavation reduce ground support of the existing structure or not?

Will groundwater lowering cause settlement.?

Will  disturbed drainage cause flooding or instability to the existing structures?

Environmentally, situations that may be investigated include impact of noise and waste that will be  generated by the proposed structure.

This type of investigation is not only limited to preconstruction it is also necessary after construction and during use of the structure.

Investigation as to the suitability and availability of materials for construction.

If the soil material on site is to be used for construction works,  support structure or both, then its suitability must be determined.

This is usually done by visual inspection and standardized tests.

Importance of soil investigation in civil engineering

Civil engineering works have some form of foundation which is supported by the ground. The interaction between a civil engineering structure and the ground that holds it is complex. Hence it is appropriate that proper soil study and investigation is carried out before structure is built in it.

It is standard practice to examine any soil on which a structure is intended to be erected in order to determine the following:

• The suitability of the soil site for the proposed work.
• Adequate and economic foundation design.
• Difficulties that may arise during construction.
• Situations that may occur after construction.

Rock types

Rock types can be classified by their method of excavation. That is, whether blasting is required, by load bearing or other physical properties.

From a civil engineer’s point of view, A rock is a solid mass composed of soil material in a very stable position of which it’s removal is only possible after blasting, or breaking by compressed air and hydraulic tools or by breaking tools such a wedges or sledge hammer.

Rocks can be placed into 3 basic categories, the are:

Igneous, sedimentary and metamorphic.

Igneous rocks include basalt and granite. They are formed by the solidification of molten material from the hot lower levels of the earth crust which have ascended towards the surface. They have very high bearing capacities about 3 times that of sedimentary rocks and 40 times that of alluvial clays and sand.

Sedimentary rocks includes, Limestone and sandstone.They were formed by stratification and cementation of earth materials over time.Their strength depends on properties such as angle of stratification and cementation as well as behaviour under wet conditions.

Metamorphic rocks are any sedimentary or igneous rock deposits which after consolidation have become changed by heat and pressure. Examples of metamorphic rocks are gneisses, slates, schists.

The most suitable type of  rock for civil engineering  foundation work is the igneous rock. Sedimentary rocks have lower  load bearing capacity due to the presence of soft clay material in their deposits. However, the load bearing capacity of all types of rock is greatly reduced as a result of weathering and earth movements.

Soil types

Types of soil includes;

• Residual soil ( too soil)
• Detrital sediments ( sands, gravel, slits)
• Organic deposits (peat)
• Calcareous deposits ( shell, coral)
• Uncemented volcanic dust.

Soil types are identified by;

• Size and nature of soil particles
• Density and structural properties

You can download the table of Field Identification and Description of Soils. (BS 5930:2015) HERE

The table can be found in page 110 of BS 5930: 2015.